Multiband travelling wave tube of reduced length capable of high power functioning

ABSTRACT

A traveling wave tube designed to operate as an amplifier within plural frequency bands, which includes a microwave line through which electrons travel and in which a signal is amplified. The microwave line includes, in succession, an input section separated from a succession of gapped output sections, each output section working within one of the operating frequency bands of the traveling wave tube. The input section, connected at one end to an input for injecting the signal to be amplified, works within a frequency band encompassing the operating frequency bands of the traveling wave tube and is intended to preamplify the signal to be amplified. The succession of output sections receives the preamplified signal, each of the output sections being intended to amplify it if the signal is at a frequency lying within its working frequency band and to let it pass through virtually without any interaction if it is at a frequency outside its working frequency band. Each of the output sections is connected at an output end to an output for extracting the preamplified signal that it has amplified. Such a traveling wave tube has a short length and is capable of operating at high power.

BACKGROUND OF THE INVENTION

The present invention relates to a multiband traveling wave tube capable of operating at high power. This tube, intended especially to be used in airborne and space applications, must be relatively short.

The development of techniques and the increasing control of materials have made it possible to develop traveling wave tubes intended to operate within a very wide frequency range and which are relatively short. These tubes are known as mini-TWTs. These are travelling wave tubes with a one-piece line in the form of a tight helix. From the frequency standpoint, a ratio of the high frequency to the low frequency of the band of at least 3 is obtained.

From the dimensional standpoint, these tubes do not exceed about thirty centimeters, but from the power standpoint they barely reach more than a few tens of watts.

It would be conceivable to extrapolate these tubes in order to increase their power but, for a given gain, achieving a higher power entails increasing the helix voltage and increasing its length. However, it is not a question of reducing the gain, so as to obtain the required power without increasing the length of the helix. This approach does not lead to a multiband traveling wave tube, of short length, capable of operating at high power.

BRIEF SUMMARY OF THE INVENTION

The subject of the present invention is a multiband traveling wave tube which has a length of the order of that of a mini-TWT but which is capable of operating at higher powers, while still maintaining a gain of the same order of magnitude.

For this purpose, the multiband traveling wave tube according to the invention comprises a microwave line along which electrons travel and in which a signal is amplified. This microwave line comprises, in succession, a microwave line input section separated from a succession of gapped microwave line output sections, each output section working within one of the operating bands of the tube.

The input section is connected, at one end, to input means for injecting the signal to be amplified and works within a frequency band encompassing the operating frequency bands of the tube. It is intended to preamplify the signal to be amplified.

The succession of output sections receives the preamplified signal, each of its output sections being intended to amplify it if it is at a frequency lying within its working frequency band and to let it through virtually without any interaction if it is at a frequency outside its working frequency band, each of the output sections being connected at one end to output means for extracting the preamplified signal that it has amplified.

Preferably, the central frequencies of the working bands of the output sections decrease with their distance from the input section.

With regard to the power of the signal amplified by an output section, this increases the further the output section is from the input section.

The microwave line sections are in the form of a helix, each helix being held in place in a sheath by dielectric supports and the various sheaths being joined together.

To be able to work in a very wide band, the input section includes dispersion-correcting means, such as gates.

Preferably, the helix of the first output section will have substantially the same length and/or the same internal diameter as the helix of the input section.

The helical wire of the first output section will also preferably have the same cross section as that of the helical wire of the input section.

To maintain, in the first output section, the synchronism, acquired in the input section, between the velocity of the electron beam and the velocity of the signal, the pitch of the helix of the first output section will preferably be smaller than that of the helix of the input section.

Preferably, the length and/or the pitch and/or the internal diameter of the helices of the output sections will increase as they get further from the input section. The same applies to the cross section of the helical wire.

To avoid the appearance of self-oscillation phenomena, the input section is provided with an attenuation region at the opposite end to that connected to the means for injecting the signal to be amplified.

For this purpose, each output section is provided with an attenuation region at the opposite end to that connected to the means for extracting the signal that it has amplified.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent on reading the description which follows of illustrative examples of tubes according to the invention, the description being illustrated by the figures which show:

FIG. 1, a schematic longitudinal section through a traveling wave tube according to the invention;

FIG. 2, an exploded schematic view of a tight-helix traveling wave tube, according to the invention;

FIGS. 3a and 3 b, cross sections of two embodiments of the input section;

FIGS. 3c, 3 d, the normalized phase velocity and the gain as a function of the frequency of the input section in FIG. 3a, respectively;

FIG. 4a, a cross section of the first output section;

FIGS. 4b, 4 c, the normalized phase velocity and the gain as a function of the frequency of the first output section in FIG. 4a, respectively;

FIG. 5a, a cross section of the last output section;

FIGS. 5b, 5 c, the normalized phase velocity and the gain as a function of the frequency of the last output section in FIG. 5a, respectively;

FIG. 6, a plot showing the power of a signal, injected into the tube of FIG. 2 and traveling along the microwave line, as a function of its frequency.

In these figures, the scales have not necessarily been respected for the sake of clarity.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows schematically a multiband traveling wave tube according to the invention.

Conventionally, it comprises, in succession in a casing 5, a gun 1 for producing an electron beam 2, a body 3 in which the electron beam 2 interacts with a signal to be amplified, and a collector 4 for collecting the electrons of the beam 2 as they leave the body 3.

According to the invention, the electron beam 2 passes, between the input 6 and the output 7 of the body 3, through a microwave line 8 formed from several gapped microwave line sections h, h₁ . . . , h_(i), . . . h_(n) placed one after another. Among these sections, the first section through which the beam 2 passes is a section h called the input section, other sections h₁ . . . h_(i) . . . h_(n) form a succession of sections called output sections and their number n is equal to the number of frequency bands B₁ . . . . B_(i) . . . B_(n) within which the tube is intended to operate, n being an integer greater than or equal to 2. Each of the output sections h₁ . . . h_(i) . . . h_(n) is intended to work in one of the operating bands of the tube B₁ . . . B_(i) . . . B_(n), respectively. Each frequency band B₁ . . . B_(i) . . . B_(n) is centered on a central frequency F₁ . . . F_(i) . . . F_(n), respectively. Each frequency band is associated with one output section.

The electron beam 2 enters each section h, h₁ . . . h_(i) . . . h_(n) via an input end e_(in) and leaves from it via an output end e_(out).

The input section h is intended to operate as a preamplifier within a very wide band B encompassing all the operating bands B₁ . . . B_(i) . . . B_(n) of the tube. The input end e_(in) of the input section h is connected to means I for injecting a signal to be amplified in the tube. Its output end e_(out) is near the input end e_(in) of the first output section h₁.

The central frequencies F₁ . . . F_(i) . . . F_(n) of the working bands B₁ . . . B_(i) . . . B_(n) of the output sections h₁ . . . h_(i) . . . h_(n) decrease with their distance from the input section h. Thus, F₁>F_(i)>F_(n).

Each output section h₁ . . . h_(i) . . . h_(n) in the succession is intended to amplify the signal preamplified in the input section h if the preamplified signal is at a frequency lying within its working frequency band B₁ . . . B_(i) . . . B_(n). The output sections h₁ . . . h_(i) . . . h_(n) have virtually no action on the preamplified signal which travels through them and which is not at a frequency lying within their working frequency band.

Each of the output ends e_(out) of the output sections h₁ . . . h_(i) . . . h_(n) is connected to output means O₁ . . . O_(i) . . . O_(n) for extracting the preamplified signal passing through it if the latter has been amplified in said output section h₁ . . . h_(i) . . . h_(n).

The signal to be amplified, injected at the input end e_(in) of the input section h, travels through it, is preamplified therein and then enters the first output section h₁. The preamplified signal which passes through the first output section h₁ is amplified therein if its frequency lies within the band B₁ and it is then extracted by the output means O₁. If the frequency of the preamplified signal is not within the band B₁, the preamplified signal travels through the first output section h₁ virtually without any coupling with the electron beam 2 and, on leaving the output end e_(out) of the first output section h₁, it enters the second output section h₂ where it is amplified if its frequency is within the band B₂ and then extracted. If its frequency is not within the band B₂, it enters the third output section h₃ and so on, from section to section, until it is amplified in the appropriate section and then extracted.

Assuming that a signal of frequency F propagates in the output section h_(i) (i being an integer between 1 and n−2), if it is not amplified therein this means that the frequency F is not included within the band B_(i). The signal of frequency F is not extracted at the output end of the output section h_(i) and it then enters the next output section h_(i+1). If its frequency F is within the frequency band B_(i+1) associated with the output section h_(i+1), it is amplified and then extracted at the end e_(out) of said section h_(i+1). If its frequency F is outside the band B_(i+1), it enters the next section h_(i+2), and so on.

An illustrative example of a tube according to the invention will now be given in greater detail. It will be assumed that this is a two-band traveling wave tube.

FIG. 2 shows such a tube seen from the outside, but the casing 5 of which has been partly opened so as to show the various microwave line sections h, h₁, h₂ which are in the form of helices. The other components inside the casing, such as the gun, the focuser and the collector, have not been shown for the sake of clarity.

The helices 20, 21, 22, each inserted into a conducting sheath 11, 11.1, 11.2, are held in place in the sheath 11, 11.1, 11.2 by means of insulating supports 12, 12.1, 12.2. In the example, three insulating supports 12, 12.1, 12.2 have been provided per helix (but only one can be seen in FIG. 2) and these supports have approximately the same length as the helix 20, 21, 22 that they hold in place. Conventionally, the supports may be made, for example, of boron nitride, alumina or beryllium oxide. There is no contact between the helices 20, 21, 22. The various sheaths 11, 11.1, 11.2 are joined together. The joints are vacuum-tight.

The input end e_(in) of the input section h is connected to input means I for injecting a signal to be amplified, these being shown in the form of a coaxial line.

The two output sections h₁, h₂ work within the band B₁, B₂, of respective central frequency F₁, F₂,respectively. The frequency F₁ is higher than the frequency F₂. The output end e_(out) of the section h₁ is connected to output means O₁ for extracting the preamplified signal if it has been amplified by said section h₁. The output means O₁ are shown by a waveguide, this being conventional at high frequency. The output end e_(out) of the section h₂ is connected to output means O₂ for extracting the signal which has been amplified by the section h₂. In the example, these means are a coaxial line. Of course, each of the input and output means could be of a different type. The section h is intended to work within a very wide band B encompassing the two bands B₁ and B₂.

A The characteristics of each of the line sections h, h₁, h₂ will now be explained in greater detail.

In order for the input section h to be able to work within the very wide band B, while still keeping the normalized phase velocity of the signal that has to be preamplified approximately constant, whatever the frequency, the input section h includes dispersion-correcting means 13 such as, for example, gates. In FIG. 3a, the gates 13 are separate from the supports 12 and are conductors extending longitudinally along the helix 20 and projecting from the sheath 11 toward the helix 20. These gates 13 are separated from the helix 20 by a space 14. They are placed between the supports 12.

Another type of gate may be used, as illustrated in FIG. 3b. These gates integrated into the dielectric supports are described in European patent EP-B-0 401 065. The helix is held in place by the dielectric supports 120 which are in turn supported by conducting elements 130 projecting from the internal wall of the sheath 11 toward the helix 20. This configuration has the advantage of obstructing the inside of the sheath less, thereby reducing the time needed to pump down and qualitatively improving the vacuum.

A ratio of the lowest central frequency F₂ to the highest central frequency F₁ of between 2 and 4 may hopefully be obtained.

FIGS. 3a and 3 b show cross sections through the input section h. The internal diameter d of the helix 20 is relatively small so that the section h can work as a preamplifier within the band B encompassing all the operating bands of the tube. This diameter depends on the frequency band to be amplified.

FIG. 3c is a diagram of the normalized phase velocity c/vφ of the signal propagating through the input section h as a function of the frequency F. It is assumed in the example described that the tube is intended to operate within two bands B₁, B₂ centered around the frequency F₀ and the frequency 3F₀, respectively. The normalized phase velocity c/vφ is the ratio of the phase velocity vφ to the velocity of light c. The solid curve is obtained in the input section h with gates 13 separate from the supports 12 and the dotted curve is that obtained without the gates 13.

FIG. 3d shows the gain G of the input section h as a function of the frequency F. The maximum gain G_(max) is obtained in the middle part of the curve, that is to say for a mid-frequency, the frequencies F₀ and 3F₀ lying on each side of the mid-frequency. In the operating bands B₁, B₂, the gain is approximately 4 to 5 dB below the maximum gain.

The first output section h₁ is that which works as an amplifier at the highest frequency, in this case 3F₀. Its operating band B₁ is narrower than the band B and the section h₁ does not require dispersion-correcting means.

FIG. 4a shows, in cross section, the first output section h₁ with the dielectric supports 12.1. To simplify matters, its helix 21 may be made with the same wire as the helix 20 of the input section h if the desired power at the output end e_(out) of the first output section is not too high. This will have approximately the same internal diameter d₁ as that d of the helix 20 of the input section h since this first output section h₁ is associated with the band B₁ whose central frequency 3F₀ is the highest.

However, its pitch p₁ may be smaller than that p of the helix 20 of the input section h in order to maintain the synchronism between the velocity of the electron beam and the velocity of the signal traveling through the helix, this synchronism being acquired in the input section h.

The length l₁ of the helix 21 is related to the gain needed to obtain the desired power at the frequency 3F₀. It is desirable for the gain of the first output section h₁ to be greater than that of the input section h. However, the length l₁ of the helix of the first output section h₁ may be of the same order as that of the helix 20 of the input section h, since the gain per unit length of a helical line without any dispersion-correcting means is greater than that of a helical line with dispersion-correcting means.

An output power of the order of a hundred watts might be expected to be achieved for a frequency of several tens of gigahertz.

FIG. 4b is a curve of the normalized phase output as a function of the frequency for this first output section h₁, while FIG. 4c is a curve of the gain as a function of the frequency. The gain is a maximum for the central frequency 3F₀.

FIG. 5a shows a cross section through the next output section h₂, which here is the last. It is associated with the band B₂ having the lowest central frequency F₀.

This second output section h₂ does not require dispersion-correcting means either, since the band B₂ is narrower than the band B. The same would apply for all the other output sections.

The internal diameter d₂ of its helix 22 is greater than that of the helix 21 of the output section h₁ which precedes it. The internal diameter of the helix varies in an approximately inversely proportional manner with the operating frequency so that the amplification parameter remains constant. The ratio of the two diameters d₁, d₂ is approximately the same as that of the corresponding central frequencies 3F₀, F₀. More generally, the internal diameter of the helices of the output sections h₁, h₂ increases with their distance from the input section h. With such a configuration, the electron beam diameter increases further on approaching the collector. As a consequence, the beam is focused in a conventional manner for a person skilled in the art.

The supports 12.2 which hold the helix 22 in place are matched to the diameter of the helix and to that of the sheath section 11.2. It is possible for the various sheath sections 11, 11.1, 11.2 not to have the same diameter.

The pitch P₂ of the helix 22 of the second output section h₂ is greater than that P₁ of the output section h₁ which precedes it, again for the purpose of maintaining the synchronism between the velocity of the electron beam and the velocity of the signal which travels through the helix 22. More generally, the pitch of the helices of the output sections increases with their distance from the input section.

It will be assumed that the signal produced by the output section h₂ has a higher power than that of the signal produced by the output section h₁ which precedes it, which means that the wire of the helix 22 has to have a larger cross section than that of the wire of the helix 21. It is possible to achieve, at the output end of the output section h₂, powers three to four times greater than those obtained at the output end of the output section h₁. Generalizing, the cross section of the wire of the helices of the output sections will increase with their distance from the input section.

The length l₂ of the helix 22 is tied to the gain needed to obtain the desired power at the frequency F₀. The section h₂ will have a length l₂ greater than that l₁ of the output section h₁ which precedes it since the frequency at which it works is lower. More generally, within the succession the length of the helices of the output sections increases with their distance from the input section.

With a length l₂ greater than the length 1 by a few centimeters, an output power of several hundreds of watts might be expected for a frequency of the order of ten gigahertz.

FIG. 5b is a curve of the normalized phase velocity as a function of the frequency for this second output section h₂, while FIG. 5c is a curve of its gain as a function of the frequency. The gain is a maximum for the central frequency F₀.

Conventionally, in order to avoid self-oscillation phenomena in the tube, an attenuation region 30, 31, 32 is provided in the microwave line sections h, h₁, h₂. More specifically, these attenuation regions cover the supports 12, 12.1, 12.2 for the helices 20, 21, 22 and they may be produced by depositing carbon, for example. These attenuation regions are respectively located with the first 30 near the output e_(out) of the input section h and the others 31, 32 near the input end e_(in) of the respective output sections h₁, h₂. The attenuation region 31 of the first output section h₁ has approximately the same length as that of the input section h. On the other hand, the attenuation region 32 of another output section h₂ is longer than that 31 of the output section h₁, which precedes it.

FIG. 6 is a curve of the power P (expressed in dBm) of a signal which is injected with an amplitude P_(in) into the tube of FIG. 2 and which travels along the microwave line until it is extracted, either at the output means O₁ or at the output means O₂.

The signal extracted at the output means O₁ has a power of amplitude P₁ and is at the frequency 3F₀. The signal extracted at the output means O₂ has a power P₂ and is at a frequency F₀. The amplitude P₂ is approximately three times greater than the amplitude P₁.

It should be noted that the powers drop rapidly at the attenuation regions 30, 31, 32, which are shown symbolically by triangles. In each output section, the signal amplified therein has an amplitude which increases strongly as soon as it propagates beyond the corresponding attenuation region 31, 32. 

What is claimed is:
 1. A traveling wave tube capable of operating as an amplifier in plural frequency bands, comprising a microwave line along which electrons travel and in which a signal is amplified, wherein the microwave line comprises, in succession, a microwave line input section separated from a succession of gapped microwave line output sections, each output section having a respective working band corresponding to one of the plural frequency bands of the traveling wave tube, in which band it behaves as an amplifier, wherein; the microwave line input section is connected at one end to input means for injecting the signal to be amplified and having a working frequency band encompassing the plural frequency bands of the traveling wave tube, in order to preamplify the signal to be amplified, and the succession of output sections receive the preamplified signal and are configured to let the frequencies lying outside its respective working band pass, virtually without any action, to a following section, each of the output sections being connected by an output end to output means for extracting the amplified signal.
 2. The traveling wave tube as claimed in claim 1, wherein the output sections are placed in an order corresponding to the working frequency bands, sections furthest from the input section having a central frequency of their operating band which is lower than closest sections.
 3. The traveling wave tube as claimed in claim 1, wherein the output sections are placed in an order corresponding to their power gain, sections having a highest gain being furthest from the input section.
 4. The traveling wave tube as claimed in claim 1, wherein the input section and output sections are in the form of helices, each helix being held in place in a conducting sheath by dielectric supports, the sheaths being joined together.
 5. The traveling wave tube as claimed in claim 4, wherein the input section includes dispersion-correcting means.
 6. The traveling wave tube as claimed in claim 5, wherein the dispersion-correcting means comprise gates.
 7. The traveling wave tube as claimed in claim 4, wherein at least one of a length and internal diameter of the helix of a first output section is substantially the same as those of the helix of the input section.
 8. The traveling wave tube as claimed in claim 4, wherein a pitch of the helix of a first output section is smaller than a pitch of the helix of the input section.
 9. The traveling wave tube as claimed in claim 4, wherein a cross-section of the helix of a first output section is substantially the same as that of the helix of the input section.
 10. The traveling wave tube as claimed in claim 4, wherein at least one of a length and pitch of the helices of the output sections increase with their distance from the input section.
 11. The traveling wave tube as claimed in claim 4, wherein an internal diameter of the helices of the output sections increases with their distance from the input section.
 12. The traveling wave tube as claimed in claim 4, wherein a cross-section of the helix of the output sections increases with its distance from the input section.
 13. The traveling wave tube as claimed in claim 1, wherein the input section is provided with an attenuation region at an opposite end to that connected to the input means for injecting the signal to be amplified.
 14. The traveling wave tube as claimed in claim 1, wherein each output section is provided with an attenuation region at an opposite end to that connected to the output means for extracting the signal that it has amplified. 